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This paper proposes a control method for medium-voltage direct-connected cascaded H-bridge multilevel inverters with model predictive control (MPC) under weak grids. The topology of this inverter and the principle of its finite control set are analyzed in this paper. Based on which, the derivation of the system predictive model is illustrated considering the weak grid. By eliminating the redundant state of the switches, the finite control set is optimized, so the control shortcomings problem of the cascaded multi-level inverter is solved for the traditional model predictive control method which means a big calculation burden and online-implement difficulty. Furthermore, the "calculate before output" method used in the past will cause the actual control output to be delayed, and in order to eliminate the delay, this paper proposes an improved MPC strategy, which introduces a two-step predictive calculation for the grid current control. Finally, the rationality and effectiveness of the proposed method are verified by Matlab/Simulink simulation.